Air shower systems of the above description have been known for many years. In the invention of U.S. Pat. No. 2,981,175 Goyette, hot or cold air is made alternatively available to a plurality of nozzles that extend along a cylinder in the cross-machine direction. The system apparently contemplates manual operation and temperature selection for each nozzle in order to control sheet caliper (thickness) in a papermaking application.
The invention of U.S. Pat. No. 3,177,799 Justus provides for controlling the diameter profile of calender rolls by moving a fluid projector unit to a selected cross-machine region of the calender roll and applying hot or cold fluid to the selected region.
The invention of U.S. Pat. No. 3,203,678 Sawyer provides an automated air shower system wherein a traversing pyrometer scans the roll or cylinder to detect hot spots in four cross-machine zones. Upon detecting temperature above a critical level in any zone the pyrometer, via its internal circuitry, activates a corresponding nozzle to direct corrective air against the roll until it is cooled below the critical temperature level.
The invention of U.S. Pat. No. 3,770,578 Spurrell provides for regulating the temperature of air flowing from each nozzle by mixing hot and cold air just behind each nozzle to desired proportions, and for simultaneously directing air from adjacent nozzles to eliminate edge effects.
The invention of U.S. Pat. No. 4,114,528 Walker provides for directing a uniform flow of air through the nozzles and adjusting the temperature of the air at each nozzle by mixing hot and cold air in a manner similar to Spurrell, above.
In the above inventions, air that is directed toward the roll or cylinder is substantially unconstrained with respect to its escape path away from the cylinder. As a result, there is substantially laminar flow along a relatively small circumferential portion of the cylinder.
In the inventions of U.S. Pat. Nos. 2,141,403 Offen, 3,190,212 Moore, 4,573,402 Sharma, and 3,266,561 Beachler, devices that are variously described as hoods or faceplates serve as a means for preventing the escape of air in directions away from the surface of the cylinder. This improves heat transfer for a given flow rate of air from the nozzles (or other means for directing air toward the cylinder) by extending the time during which the air is in contact with the surface of the cylinder, and by creating conditions under which air between the hood and the cylinder can be maintained in a turbulent state. U.S. Pat. No. 4,573,402 Sharma employs electrical heating elements within an air plenum of which the hood or faceplate is a part, and also discloses the idea of directing air toward the cylinder at an angle whereby the directed air has a component of velocity that is in a direction opposite the direction of rotation of the cylinder.
U.S. Pat. No. 2,141,403 Offen discloses a hooded air shower system that provides two end seals for preventing the escape of air between the hood and a web into the surrounding atmosphere, and provides exhaust openings in the hood as escape routes for the discharged air.
The above inventions that employ hoods have been developed under at least two tacit assumptions from which the instant invention departs.
First, all have the feature that the hood is used as a manifold through which air is directed toward the cylinder. While this feature may be used in an embodiment of the instant invention, this invention recognizes that the feature is unnecessary and may be undesirable because it reduces the available circumferential range of the cylinder over which a given portion of air can travel and over which heat transfer can take place.
Second, all have the feature that when air is directed toward the cylinder, substantial portions thereof are permitted to flow in either direction along the cylinder. This feature is costly because it requires higher flow rates to achieve a given rate of heat transfer than would be required if movement of the air along the cylinder were substantially restricted to one direction.
Further distinguishing features of the invention will be recognized in the drawings and description which follow.